The selective recognition and activation of small molecules, a process frequently mediated by metalloproteins, is at the heart of many crucial biological systems. The ability to understand and selectively modify metalloprotein reactivity would have important implications for the rational development of therapeutic agents. Complete understanding of metalloprotein reactivity is dependent, however, on the availability of an accurate structural description of the metal active site. The long-range objective of this proposal is determination of the metal-site structure for a series of metalloenzymes which mediate the reactivity of the 0/2/n- unit. This will be accomplished by means of extended X-ray absorption fine structure (EXAFS) spectroscopy and differential anomalous X-ray scattering (DAS). EXAFS allows precise determination of the immediate coordination environment (bond length, coordination number and ligand identity) while DAS provides information about more distant (3-15 A) metal-metal interactions. The systems to be studied represent several different enzymatic categories, however for each system the reactivity of the metal center with the 0/2/n- unit is crucial to the biological function. The lack of suitable single crystals precludes the use of X-ray crystallography to study these systems and other physical techniques are unable to provide the detailed structural information which can be obtained from EXAFS and DAS. The systems to be studied are the blue copper oxidase laccase, a non-heme iron oxygenase which catalyzes the oxidation of phthalate, a Mn containing pseudocatalase from Lactobacillus plantarum, the Mn substituted derivative of horseradish peroxidase, and the Mn cluster from the photosynthetic oxygen evolving complex. Structural comparisions of these different systems should permit a more complete understanding of the essential features of metalloenzyme mediated activation of the 0/2/n- unit.